EP0559043A1 - Method for heat exchanger control - Google Patents

Abstract

A description is given of a method for heat exchanger (3) control which is characterised in that the enthalpy flux in the heat exchanger (3) serves as the manipulated variable. The robust control, which can be used in a versatile fashion, is particularly suited for use in endothermic reactions and in distillation. <IMAGE>

Description

The invention relates to methods for controlling heat exchangers for setting a controlled variable on the process side of the heat exchanger.

The invention is in the field of process engineering operations that require a regulated supply of energy. These energy consumers are mostly supplied centrally from a power plant on an industrial scale. The water vapor produced by the power plant is directed to the individual consumers via a so-called steam rail. In the rarest cases, the incoming steam is sent directly to the process by the consumer. An intermediate heat exchanger (heat exchanger) predominantly transfers only the heat contained in the water vapor to the process. The control of the amount of energy transferred forms the subject of the present invention.

The process used on an industrial scale can be described with reference to FIG. 1. For example, in order to regulate the temperature in a process (TC), the actual manipulated variable, the energy transferred, is translated into a substitute manipulated variable, namely the amount of steam conducted into the heat exchanger 3. This is shown in the drawing by the control unit FC (feed control). It controls the valve 2. As a control value, the supply control mostly serves the pressure drop of the steam via a measuring orifice 5 within the feed line. Under ideal conditions, i.e. at constant pressure and temperature, the volume flow and therefore also the mass flow through the orifice is proportional to the root of the pressure drop at the orifice (orifice equation).

The disadvantage of this known method, however, is that the conditions (pressure, temperature) within a steam rail, which feeds several consumers from the power plant, can change. For example, disturbances are already generated by switching consumers on and off along a steam rail.

The task was therefore to develop a regulation for better control of the amount of heat entered in a process.

• a) Messung des Massenstromes (ṁ) und mindestens einer der Größen Druck (p) und Temperatur (T) des Betriebsmittels in der Zufuhrleitung (1) zum Wärmeübertrager (3);
• b) Bestimmung des Enthalpiestromes in den Wärmeübertrager (3) unter Verwendung der in a) gemessenen Größen;
• c) Verwendung des durch b) bestimmten Enthalpiestromes in den Wärmeübertrager (3) als Stellgröße für die Regelgröße auf der Prozeßseite, wobei der Wärmeübertrager (3) durch Veränderung des in ihn fließenden Enthalpiestromes nach Maßgabe der Regelgröße gesteuert wird.

The task was solved by a procedure by performing the following steps;

• a) measurement of the mass flow (ṁ) and at least one of the variables pressure (p) and temperature (T) of the operating medium in the supply line (1) to the heat exchanger (3);
• b) determination of the enthalpy flow into the heat exchanger (3) using the quantities measured in a);
• c) Use of the enthalpy flow determined by b) in the heat exchanger (3) as a manipulated variable for the controlled variable on the process side, the heat exchanger (3) being controlled by changing the enthalpy flow flowing into it in accordance with the controlled variable.

The measurement of the variables pressure (p), temperature (T) and the determination of the mass flow (ṁ) are carried out by known methods and devices. It should be noted, however, that the usual methods for determining the mass flow (throttle, inductive flow measurement) only determine a volume flow, which can only be converted into a mass flow after multiplication by the density (δ) of the working medium. This conversion can lead to inaccuracies if it is not taken into account that δ depends on T and p. The enthalpy flow into the heat exchanger is obtained by multiplying the mass flow (ṁ) by the specific enthalpy (h _i ) of the working medium, which can be calculated in a known manner if the two state variables p and T are known. If the working medium is For example, water vapor, but exactly at a phase boundary, using one of the known thermodynamic relationships, the determination of one of the two variables, pressure or temperature, is sufficient to obtain the other state variables, including the spec. Enthalpy. This is the case, for example, if water vapor in the saturated state is used as the working medium.

If the enthalpy current (Ė) is known, this can be used as a manipulated variable. The actual controlled variable, for example the temperature in a process engineering process, can be changed by a certain amount if the enthalpy flow into the heat exchanger is increased or decreased by an amount corresponding to this change. In the simplest case, this is done by a supply control (FC), in which the mass flow into the heat exchanger is controlled with the aid of a valve.

The new method enables precise, direct control of the amount of energy fed into a process. The new control procedure takes into account the interference to which the working medium in the feed line (steam rail) is subjected. This prevents these interferences from the actual process (distillation etc.). This has the advantage that this process can be controlled within narrow and more precisely definable tolerances. In addition, the amount of heat produced is used more economically.

The process can be used with advantage in technical distillation or rectification columns with diameters over 0.150 m, preferably over 0.5 m, in particular over 1.0 m. Due to their inertia, these technical columns require particularly precise control.

A preferred control variable on the process side is the temperature, since this is the most important process variable in the systems mentioned. However, other variables such as the sump level can also be controlled with the new process.

A preferred embodiment of the new method consists in taking into account the enthalpy flow leaving the heat exchanger again. This can be achieved by subtracting the specific enthalpy (h _o ) from the specific enthalpy of the supplied working medium (h _i ) and multiplying this difference by the mass flow (ṁ). It is assumed that there are no mass losses in the heat exchanger. If condensate is present, it is sufficient to only measure the temperature of the condensate to determine h _o .

gleichgesetzt wurde, müssen im dynamischen Fall also Größen wie bevorzugterweise die Totzeit ⊖ und die Zeitkonstante τ des Wärmetauschers sowie das zeitliche Verhalten der Austrittstemperatur T_o berücksichtigt werden. Einen besonders bevorzugten Ansatz zur Bestimmung des übertragenen Enthalpiestromes liefert

Dabei bedeutet

E :

Übertragene Enthalpie

t :

Zeit

⊖ :

Totzeit

τ :

Zeitkonstante

τ_b :

rechnerische Verweilzeit des Heizmittels im Wärmetauscher

K₁,K₂:

Apparate (Wärmetauscher)-Konstanten (rechnerisch oder experimentell bestimmbar)

V :

Flüssigkeitsvolumen des Wärmetauschers auf der Heizseite

V̇_k :

Volumenstrom des Kondensats

ṁ :

Massenstrom des Heizmittels (Dampf)

h :

spezifische Enthalpie des Dampfes/Kondensats

T :

Temperatur in °C (oder K)

c :

spezifische Wärme des Kondensats

Indizes:

i : Wärmetauscher Eingang
o : Wärmetauscher Ausgang (Kondensat)

Der Enthalpiestrom läßt sich also mittels geeignet gewählter elektronischer Bausteine wie Multiplizierern und Differenziergliedern aus meßbaren Größen bestimmen. Ebenso sind für diese Aufgabe programmierbare Mikroprozessoren anwendbar.Another preferred variant of the method also takes into account the temporal or dynamic behavior of the heat transfer process. The dynamic behavior is determined by determining the transfer function. For this purpose, for example, the input temperature of the working medium can have a defined course over time, such as a step function or a periodic function or a combination of both, are imprinted and the behavior of the heat exchanger is determined by measuring the time profile of the temperature T _{o of} the working medium at the outlet of the heat exchanger. This behavior depends, among other things, on the throughput and the residence time characteristics of the mass flow in the heat exchanger and on the heat capacity of the heat exchanger. In contrast to the static case described above, where the enthalpy flow E transferred with $\dot{\text{m}}{\text{·(H}}_{\text{i}}{\text{-H}}_{\text{O}}\text{)}$

equated, in the dynamic case, variables such as preferably the dead time ⊖ and the time constant τ of the heat exchanger and the time behavior of the outlet temperature T _o must be taken into account. A particularly preferred approach for the determination of the enthalpy current transferred provides

Here means

E:

Transmitted enthalpy

t:

time

⊖:

Dead time

τ:

Time constant

τ _b :

calculated residence time of the heating medium in the heat exchanger

K₁, K₂:

Apparatus (heat exchanger) constants (can be determined by calculation or experiment)

V:

Liquid volume of the heat exchanger on the heating side

V̇ _k :

Volume flow of the condensate

ṁ:

Mass flow of the heating medium (steam)

H :

specific enthalpy of the steam / condensate

T:

Temperature in ° C (or K)

c:

specific heat of the condensate

Indices:

i: Heat exchanger input
o: heat exchanger outlet (condensate)

The enthalpy current can therefore be determined from measurable quantities by means of suitably selected electronic components such as multipliers and differentiators. Programmable microprocessors can also be used for this task.

Despite more complex control electronics, it has. Implementation form the advantage of being able to react more precisely to rapid flow disturbances of the working medium.

The exact knowledge of the enthalpy flow transferred in the heat exchanger is used in a preferred variant of the method in order to obtain a control system for monitoring the heat transfer behavior of the system.

dargestellt werden. Dabei ist ΔT die Temperaturdifferenz zwischen der Temperatur des Arbeitsmediums und der Temperatur des wärmegespeisten Prozesses, also beispielsweise die Temperatur im Sumpf einer Destillieranlage. A ist ein Maß für die Fläche, durch die der Wärmetransport stattfindet, und k ist der Wärmedurchgangskoeffizient. Durch Vergleich der übertragenen Enthalpiemenge mit Änderung der Temperaturdifferenz ΔT läßt sich (A als konstant angenommen) eine Änderung, beispielsweise eine Verschlechterung des Koeffizienten k feststellen.The enthalpy transferred, ie the amount of heat introduced into the process, can be used as a product $\text{k · A · ΔT}$

being represented. Here, ΔT is the temperature difference between the temperature of the working medium and the temperature of the heat-fed process, for example the temperature in the bottom of a still. A is a measure of the area through which the heat transfer takes place, and k is the heat transfer coefficient. By comparing the amount of enthalpy transferred with a change in the temperature difference ΔT, a change (A assumed to be constant) can be determined, for example a deterioration in the coefficient k.

By monitoring the heat transfer behavior, e.g. a deterioration, also called fouling, can be controlled, for example, by switching to cleaned heat exchangers or switching off for cleaning.

Figur 1 und Figur 2

das bekannte Schema zur Temperaturregelung eines Prozesses mittels eines dampfgespeisten Wärmeübertragers.

Figur 3

zeigt die Temperaturregelung des gleichen Prozesses nach dem neuen Verfahren.

The invention is explained below by way of example with reference to the drawing. Show

Figure 1 and Figure 2

the known scheme for temperature control of a process using a steam-fed heat exchanger.

Figure 3

shows the temperature control of the same process using the new method.

As already described, according to the known methods, the setting of a certain temperature requires regulation of the supply quantity FC by means of a valve 2 as a function of the measured mass flow through the steam line 1. In the case described, the mass flow is measured by measuring the pressure difference Δp at the orifice 5, which sits in the steam line 1, made. The amount of steam thus regulated enters the heat exchanger 3 and causes the desired temperature change in the process.

According to the new method (here the implementation, without taking into account the dynamic behavior of the heat exchanger, but taking into account the enthalpy flow discharged through the condensate), instead of translating the temperature control variable into the supply quantity control variable, the translation into the enthalpy flow control variable EC (enthalpy control). A deviation from the setpoint causes the enthalpy control EC to track the valve 2. The EC obtains the required measured values by measuring the pressure P _i and the temperature T _i in the steam feed 1. The specific enthalpy h _i and the density δ can be determined from these measured variables determined by known electronic components (multipliers, etc.) or programmable microprocessors. One way of doing this is provided by calculation sheets known to the person skilled in the art (for example VDI Warmth Atlas, 6th edition, 1991, DB 1 to DB 15), the tabulated values can be used as support points for interpolation of intermediate values. The mass flow is determined in the known manner with the aid of an orifice 5. The pressure drop Δp is measured over the orifice. The volume flow is first determined from this measured value on the basis of the known aperture equation. The mass flow ṁ is then determined using the state variable δ, the density of the working medium, determined from the quantities P _i and T _i . To determine the enthalpy flow leaving the heat exchanger 3, it is sufficient to measure the temperature T _{o of} the condensate. The enthalpy transferred in the heat exchanger 3 results from the formation of the product of the mass flow ṁ and the difference between the specific enthalpies h _i , h _o .

Claims (7)

Control method for a heat exchanger (3), comprising a process side and an operating medium side as well as a supply line (1) for the gaseous operating medium, for setting a control variable on the process side, characterized by the following steps: a) Measurement of the mass flow (ṁ) and at least one of the quantities pressure (p) and temperature (T) of the equipment in the supply line (1) to the heat exchanger (3): b) Determination of the enthalpy flow into the heat exchanger (3) using the variables measured in a): c) Use of the enthalpy flow determined by b) in the heat exchanger (3) as a manipulated variable for the controlled variable on the process side, the heat exchanger (3) being controlled by changing the enthalpy flow flowing into it in accordance with the controlled variable. Method according to Claim 1, characterized in that the difference between the enthalpy flow into the heat exchanger (3) and the enthalpy flow leaving the heat exchanger (3) is used as the manipulated variable for regulating the heat transfer. Method according to one of claims 1 to 2, characterized in that the temporal behavior of the heat exchanger (3) is taken into account when determining the enthalpy flow. Method according to one of claims 1 to 3, characterized in that the enthalpy flow is changed by changing the mass flow (ṁ). Method according to one of claims 1 to 4, characterized in that the temperature is used as the control variable on the process side. Method according to one of claims 1 to 5, characterized in that the enthalpy flow transmitted by the heat exchanger (3) is compared with the temperature difference between the gaseous medium and the process side and this comparison serves as a measure of the change in the heat transfer properties of the heat exchanger (3). Method according to one of claims 1 to 6, characterized in that the gaseous medium is water vapor.

Images